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-   -   Dimensionless equations, variables, mesh etc (http://www.cfd-online.com/Forums/openfoam-solving/66283-dimensionless-equations-variables-mesh-etc.html)

 bhh July 10, 2009 09:56

Dimensionless equations, variables, mesh etc

Hi,

All my equations, variables and coordinates etc. are dimensionless. OF require dimensions on all variables, coordinates etc.. How can I do my modelling in OF without dimensions?

Regards

Bjorn

 titio July 10, 2009 11:33

bhh,

Even with normalized dimensions, you still have the points relative position. From there you create the mesh. Concerning the variables, it is a question of defining dimensioned scalar, or volVectorField, or equivalent, in the createFields.H fil.

Regards,

Titio

 bhh July 12, 2009 23:56

Hi,

How and where may I turn off the dimension checking?

Bjørn

 isabel July 13, 2009 04:14

I need to turn off the dimension checking. Does anybody know how to do it in OpenFOAM?

 henrik July 13, 2009 09:22

Dear All,

google("dimension checking openfoam") yields

http://openfoamwiki.net/index.php/Ma...al_checking.3F

as the first hit - not that I think it's a good idea to switch it off!

Henrik

 bhh July 13, 2009 10:41

Henrik,

Thanks for the info!

I agree that the dimension checking should generally not be turned off. However, when doing a non-dimensional problem it should be OK, or?

Bjorn

 henrik July 13, 2009 11:31

Dear Bjorn,

Why do you want to implement the equation set in dimensionless-form in the first place? You loose one of the best features of OpenFOAM while you need it most, i.e. while implementing a new set of equations and solution algorithmn.

Without knowing you and your skills, my bet is that you will make mistakes which could have been easily flagged through dimension checking. I cannot tell you how much I hated the void fraction for being dimensionless, but that one cannot be helped ;-)

I would rather read the dimensionless parameters from dictionary and set the properties in the code. Post-processing can do the reverse and everybody is happy.

Henrik

 sven August 5, 2009 12:21

Hey henrik,

where can I set dimensions, when I implement new equations, that means in which file can I set the dimensions of constants that come with the implementation of a new equation? Thank you!

 henrik August 5, 2009 12:41

Dear Sven,

the only dimensions that are not read in from file are those that arise from the discretisation (fvc::* and fvm::* methods). Those can not be changed - only the errors can be switched off.

Not sure this answers your question ...

Henrik

 sven August 5, 2009 13:09

Hey Henrik,

thanks for your answer. If I understand you right, you say that the dimensions of constants (for example Cmu in some turbulence models) are read in from file. I agree with that. But in the files where these constants are defined, there is no information about the dimensions of these constants. Thus, I guess there must be another file in which the dimensions of these constants are set.
I also tried to switch of the dimensions checking of OF. Unfortunately I can not find the controlDict file. According to the user Guide it should be located in \$WM_PROJECT_DIR/OpenFOAM-1.5/.OpenFOAM-1.5. But in WM_PROJECT_DIR/OpenFOAM-1.5 there is no hidden folder named .OpenFOAM-1.5.
Thanks for your help!

 henrik August 5, 2009 14:01

Sven,

Cmu, C1, C2, alphaEps are dimensionless.

The guide is outdated: The controlDict is now in \$WM_PROJECT_DIR/etc

Please file a bug on this or make a note on the Wiki.

Henrik

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